The Purcellville teen was in the throes of depression Saturday afternoon when he began cutting himself at a friend’s house, his mother said.

The friend called 911 seeking help. But in the chaotic minutes that followed, 17-year-old Christian Alberto Sierra would die at the hands of a Purcellville officer summoned to save him.

Virginia State Police, who are investigating the case, said that Sierra, armed with a knife, lunged at an officer, who then shot him.

Sandra Sierra, Christian’s mother, wondered why police used such force on a teen they knew was in emotional distress, not committing a crime.

“Why would you shoot a child that is suicidal?” she asked. “You are there to save him, not finish him off.”

Christian Sierra’s mother and witnesses to the shooting said the final moments of the Loudoun Valley High School junior were a jumble of action as friends and strangers desperately tried to help the teen, who was acting erratically.

Sandra Sierra said she has been told by eyewitnesses that her son was watching movies with friends when he became depressed. She said she was unsure what triggered the episode.

Christian began cutting himself with a knife, his mother said, and a friend called 911 shortly after 2 p.m.

Sandra Sierra said the call worried her son, who fled the home in the 100 block of Frazer Drive by jumping off a balcony. A friend gave chase.

Kelly Sobel, a neighbor several doors down, was getting out of the shower when she heard her neighbor scream: “Stop! Stop!”

Sobel glanced out the window and saw the neighbor running down her front steps with a dish towel. Sandra Sierra said her son was bloody at that point.

Sobel said that a second later, she heard Christian Sierra and the friend chasing him barrel into her garage door with a loud boom.

The friend and her neighbor tried to calm Sierra, but he took off again, Sobel said. The friend and neighbor followed, she said. “They were flying down the street,” Sobel said.

Christian Sierra rounded a corner. Several witnesses said they heard a police officer yell “Freeze!” or “Stop!” and then heard shots. Sobel said she heard three or four pops.

State police said Christian Sierra was immediately treated and transported to a hospital, where he died. The officer was not injured.

State and Purcellville police declined to provide additional details or name the officer involved but said the officer had been placed on administrative leave pending the completion of the investigation.

“This has been a very tragic situation for the community, the family and our officers,” said Purcellville Police Chief Darryl Smith. “We come to work to do the job of protecting and serving.”

Smith said it was the first fatal shooting by an officer in the line of duty in the history of the department, which has 15 officers.

Paige Holdeman, 16, a friend of Sierra’s, said the hallways at Loudoun Valley were quiet Tuesday. “Some people were trying to keep it to themselves, and other people were bawling their eyes out,” she said.

The sophomore said she met Sierra in a math class the year before and they became friends quickly.

“When you were around him, he put off really good vibes, nothing negative,” she said.

“He always came off as just a really happy fun guy,” she said. “You would never picture this out of Christian.”

Sandra Sierra said her son suffered from depression but was not violent. He had been on the wrestling team before this year.

A vigil was held for Christian Sierra on Sunday night. At the scene of the shooting Tuesday, candles and a teddy bear were saturated by a heavy downpour.

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